This invention is related to high-purity silicon nitride powder and its manufacturing methods. As well known, silicon nitride has the characteristics of:(1) high mechanical strength at elevated temperatures, (2) good resistance to the thermal shock and corrosion, (3) moderate thermal conductivities, (4) low thermal expansion coefficient, (5) higher chemical stability, and (6) high electric insulation.
Accordingly, silicon nitride has various uses, and as, for metal refining, ceramic industry, machine industry as refractory materials, corrosion resistance materials and insulators.
In recent years, silicon nitride has been given considerably higher interests as a high-performance ceramic material for gas turbines required for higher strength and resistance to thermal shock and corrosion at elevated temperatures.
Thermal and mechanical properties which are important factors as silicon nitride ceramics are greatly influenced by purity, crystal phase, particle size and particle shape of the raw material (silicon nitride).
Especially, fine and high purity alpha-phase nitride powder is required in order to satisfy the above-mentioned characteristics. Silicon nitride powder is generally manufactured by the following methods:
(1) By heating silica and graphite powder in a nitrogen atmosphere, silica powder being reduced by the said graphite powder to the active silicon. The silicon reacts with nitrogen.
This is generally called "silica reduction method". In this method, however, the reacted products are mixtures of alpha-silicon nitride, beta-silicon nitride, silicon oxynitride and silicon carbide. It is difficult to obtain high purity alpha phase at any time in this method.
(2) Green compact of silicon powder is heated in the current of nitrogen or ammonia, during which, the nitrogen-gas pressure is controlled at the temperature less than 1500.degree. C.
The obtained silicon nitride by this method contains much beta phase silicon nitride and it is difficult to obtain fine powder in this method. Additionally, pulverization is necessary for a long time for obtaining fine powder, and addition of contamination to the product is inevitable in the pulverizing process. Thus the product is not suitable for the raw material in fabricating high-strength silicon nitride ceramics.
(3) As another method, silicon nitride is obtained by vapor-phase reaction of silicon halide with ammonia. By this method, it is possible to obtain high-purity alpha silicon nitride, however, the method is restricted for the preparation of nitride film, and is not suitable for the manufacture of silicon nitride powder.
(4) Whereas, in the method of thermal decomposition of silicon imide obtained by the conventional method in an inert atmosphere as nitrogen or argon atmosphere, fine silicon nitride powder can be easily obtained, for instance, in the use of silicon imide obtained by the conventional reaction of silicon halide with ammonia, it is inevitable to contain chloride as an impurity to the product, and in fabricating the sintered body of the silicon nitride, the essential defect is to disturb the densification of the body.
The above inventors have accomplished this invention as a result of earnest investigations for the production of high-purity silicon nitride powder after improvement of the above defects.
In other words, this invention provides high-purity silicon nitride powder which is suitable as a raw material of high density and high-purity silicon nitride ceramics, with higher content of nitrogen and very low content of chlorine and its manufacturing methods.
Detailed explanation of this invention will be made hereinafter. High-purity silicon nitride powder obtained by this invention contains chlorine only less than 0.05 weight percent and nitrogen more than 38 weight percent.
In fabricating silicon nitride ceramics in the use of the above silicon nitride powder, it is possible to obtain high density and high-strength silicon nitride ceramics at elevated temperatures.
The manufacturing method by this invention is as follows:
After heat-treatment of a nitrogen containing silicon compound in an ammonia-atmosphere, the obtained powder in the treatment is calcinated for obtaining silicon nitride as feature of this invention.
Here, nitrogen-containing silicon compounds used in this invention are tetra-amide mono-silane (Si(NH.sub.2).sub.4) and silicon imide Si(NH).sub.2. These nitrogen-containing silane compounds used as raw materials can be manufactured by (1) reaction of gaseous silicon tetrachloride with solid ammonia at a low temperature, (2) reaction of solid silicon tetrachloride with gaseous ammonia at a low temperature, (3) reaction of silicon tetrachloride dissolved in n-hexane, and gaseous ammonia at 0.degree. C. for obtaining Si (NH).sub.2.
Nitrogen-containing silane compounds manufactured by these methods can all be used as raw materials for this invention. However, in order to obtain very fine and high-purity silicon nitride powder of this invention, it is desirable to use nitrogen-containing silane compounds manufactured by the following synthetic method. That is, gaseous silicon tetrachloride and gaseous ammonia are introduced into reaction zone in the inert gas-atmosphere, such as nitrogen or argon atmosphere.
These reactants such as SiCl.sub.4 and NH.sub.3 react at the temperatures over -30.degree. C. and less than 70.degree. C., preferably 0.degree.-70.degree. C. or more preferably 10.degree.-30.degree. C. The mixture of silane compounds obtained by this synthetic method and by-produced ammonium chloride, or silane compounds eliminating ammonium chloride can be especially favorable in the use as a raw materials for this invention.
The reactions of SiCl.sub.4 with NH.sub.3 is done at about R. T. as above mentioned, thus there is no absorption of ammonia to the produced silane compounds. Also, the reaction-rate is high, thereby, it is sufficient to use stoichiometric amount of SiCl.sub.4 and NH.sub.3. Of course, these reactants may be introduced into the reacting zone continuously for the synthesis of silane compounds.
Sufficient time is necessary for contacting SiCl.sub.4 with ammonia for the preparation of silane compounds, while these reactants react rather promptly at the above temperature for the preparation of silane compounds. A feature of this invention is to heat silane compounds obtained in the above synthetic method in an ammonia atmosphere.
An ammonia atmosphere in this invention means that of ammonia alone or the mixture of ammonia and inert gases such as ammonia and argon, ammonia and nitrogen, or ammonia and hydrogen.
In the use of the mixed gas of ammonia and other gases, the ratio of the mixture is not restricted particularly, however, it is desirable to use excessive gaseous ammonia.
The heat-treatment of nitrogen containing silane compounds obtained by the above method in an ammonia atmosphere should be done at the temperatures over 400.degree. C., preferably over 600.degree. C. by supplying the gas containing ammonia. At the temperatures less than 400.degree. C. for the heat-treatment, it becomes incomplete to remove chlorine, combined in silane compounds, and it is unfavorable to contain the combined chlorine in silicon nitride powder. In this case, a longer heat-treatment time is necessary at lower temperatures and rather shorter time is enough at higher temperatures.
Generally, the standard time is that up to the stoppage of formation of by-products such as hydrogen chloride by the heat-treatment. Moreover, during heat-treatment, it is necessary to supply the sufficient amount of ammonia consumed for thermal decomposition of silane compounds. It is also desirable to circulate the gas containing ammonia in the reaction zone for removing the by-products. The maximum heat-treatment temperature of nitrogen containing silane compounds and later mentioning calcined temperature are not particularly restricted, and yet, it is undesirable to use higher temperatures over the necessary levels in view of saving energy.
Amorphous silicon nitride can be obtained by calcination of the powder obtained by the above heat-treatment in an atmosphere as it is, or in an inert atmosphere, at the temperature in the range of 1000.degree. to 1200.degree. C., while alpha-phase silicon nitride (over 95% of alpha-phase content) is obtained by calcination at a temperature in the range from 1200.degree. to 1600.degree. C. Also, beta phase silicon nitride can be obtained by calcination at the temperatures over 1600.degree. C., respectively.
It is possible to make heat-treatment and calcination of silane compounds at the same temperature and at the same time in the same reaction zone. As aforementioned, it is possible to obtain silicon nitride having each special crystal phase by adjusting the calcination temperature, while the calcination temperature is not specially limited.
In the heat-temperature of nitrogen-containing silane compounds in this invention, it is desirable to use the furnace made of non-oxide material, e.g., silicon nitride, silicon carbide or carbon, in order to manufacture pure silicon nitride powder in this invention.
It is difficult to achieve the objective of this invention in the use of the furnace made of quartz, glass, alumina, mullite, magnesia, etc. By the above conditions, it is desirable to obtain high-purity silicon nitride powder over 38% content of nitrogen, and chlorine less than 0.05 weight percent.
Accordingly, when silicon nitride powder is used as a raw material for the sintered body, the body is useful as a structural component for gas-turbines, etc. which need higher strength, chemical and physical stability.